Black Box - The 1991 New, Easy to Master Dungeons & Dragons Game, the fourth revision of Basic D&D, served as introductory set to the Rules Cyclopedia and covered levels 1-5. Sometimes used for all Basic D&D sets from 1991 to 1999, though not all came in black boxes.

Blue Box - The first Basic D&D set, published in 1977 and edited by Eric J. Holmes. Covered levels 1-3 and was more closely related to Original D&D than later sets. Also the only edition in which the magic-user spell Magic Missile required a to-hit roll.

B/X - The second revision of Basic D&D, included the Tom Moldvay edited Basic Set and the Expert Set edited by Dave Cook and Steve Marsh. Covers level 1-14 and was only published from 1981 to 1982.

C

C&C - Castles and Crusades.

Castles & Crusades - A rules-lite RPG based on the SIEGE engine system, which uses many of the systems from 3E D&D.

Classic D&D - Can mean Basic D&D, or Basic D&D and 0E.

D

D20STL - d20 System Trademark License, a license from Wizards of the Coast allowing third-party vendors to use certain trademarks and logos. Revoked by Wizards of the Coast in 2008 and replaced with the GSL (Game System License).

DCC RPG - Dungeon Crawl Classics RPG, an upcoming RPG that is not a retro-clone, more a reimagining of D&D if it was created with access to the OGL back in 1974.

DMG - Dungeon Masters Guide

Dungeonslayers - A rules-lite RPG that is free and released under a Creative Commons License.

E

Endgame - The point in D&D where the characters retire from adventuring to build castles and strongholds, engaging in diplomacy and war while running their "kingdom", be it a keep on the borderlands, a wizard's tower, or a new thief's guild in town.

F

FF - Fiend Folio

FLGS - Friendly Local Game Store, a dying breed of retailer.

FNGS - Friendly Neighborhood Game Store, not as commonly used as FLGS.

G

GSL - Game System License, successor to the D20STL and OGL. Much more restrictive than it's predecessors.

L

Labyrinth Lord - A retro-clone of the B/X D&D rule set.

LBB's - The Little Brown Books, a reference to the three booklets that made up 0E D&D. The parchment-colored, digest sized booklets are Men & Magic, Monsters & Treasure, and The Underworld and Wilderness Adventures.

LL - Labyrinth Lord

M

Megadungeon - A dungeon so large, and constantly changing, it could challenge a party from 1st level through to the endgame. While Gary Gygax and Dave Arneson both created megadungeons for their home campaigns (Castle Greyhawk and Castle Blackmoor), neither was ever published.

Microlite20 - A simplified, rules-lite version of the D20 system.

Microlite74 - A simplified, rules-lite version of the D20 system that recreates the feel and style of 0E D&D. The 74 in Microlite74 is a reference to the year 0E D&D was first published, 1974.

MM - Monster Manual.

MM2 - Monster Manual 2.

O

OA - Oriental Adventures

OD&D - Original Dungeons and Dragons, another name for 0E D&D. Sometimes the Holmes set is included.

OEC - Original Edition Characters, a supplement to Labyrinth Lord that provides rules for characters from 0E D&D in place of the default B/X D&D characters in Labyrinth Lord.

OGC - Open Game Content, any material release under the Open Gaming License (OGL).

OGL - Open Gaming License, an open-source license released by Wizards of the Coast in 2000. While it was commonly used with the D20STL up until 2003, it's now used mainly to publish retro-clones of previous editions of D&D.

OPD - One Page Dungeon

OSR - Old School Renaissance, a surge in popularity of previous editions of D&D. Common themes are are megadungeons, sandbox campaigns, and an emphasis on player rather than character skills. Matthew J. Finch's A Quick Primer for Old School Gamingarticulates many of these concepts quite well. The "R" is OSR can also be defined as Revolution, Resurgence, Restoration, Rules, Rebellion, Reformation, or Resistance, though Renaissance is commonly accepted.

OSRIC - Old School Reference & Index Compilation, a retro-clone of 1E AD&D.

P

PHB - Players Handbook.

R

RC - Rules Cyclopedia.

Retro-Clone - A replica of an out-of-print game, usually based on a previous version of D&D, but not limited to that. Many retro-clones take advantage of the OGL.